Scientists may have solved the mystery of how human hands became nimble enough to make and manipulate stone tools.
The team reports in the journal Evolution that changes in our hands and fingers were a side-effect of changes in the shape of our feet.
This, they say, shows that the capacity to stand and walk on two feet is intrinsically linked to the emergence of stone tool technology.
The scientists used a mathematical model to simulate the changes.
Other researchers, though, have questioned this approach.
Campbell Rolian, a scientist from the University of Calgary in Canada who led the study, said: "This goes back to Darwin's The Descent of Man.
"[Charles Darwin] was among the first to consider the relationship between stone tool technology and bipedalism.
"His idea was that they were separate events and they happened sequentially - that bipedalism freed the hand to evolve for other purposes.
"What we showed was that the changes in the hand and foot are similar developments... and changes in one would have side-effects manifesting in the other."
To study this, Dr Rolian and his colleagues took measurements from the hands and feet of humans and of chimpanzees.
Their aim was to find out how the hands and feet of our more chimp-like ancestors would have evolved.
The researchers' measurements showed a strong correlation between similar parts of the hand and foot. "So, if you have a long big toe, you tend have a long thumb," Dr Rolian explained.
"One reason fingers and toes may be so strongly correlated is that they share a similar genetic and developmental 'blueprint', and small changes to this blueprint can affect the hand and foot in parallel," he said.
With this anatomical data, the researchers were able to create their mathematical simulation of evolutionary change.
"We used the mathematical model to simulate the evolutionary pressures on the hands and feet," Dr Rolian explained.
This model essentially adjusted the shape of the hands or the feet, recreating single, small evolutionary changes to see what effect they had.
By simulating this evolutionary shape-shifting, the team found that changes in the feet caused parallel changes in the hands, especially in the relative proportions of the fingers and toes.
These parallel changes or side-effects, said Dr Rolian, may have been an important evolutionary stem that allowed human ancestors, including Neanderthals, to develop the dexterity for stone tool technology.
Robin Crompton, professor of anatomy at the UK's Liverpool University, said the study was very interesting but also raised some questions.
"I am not personally convinced that the foot and hand of chimpanzees are a good model [of human ancestors' hands and feet] - the foot of the lowland gorilla may be more interesting in this respect," he told BBC News.
He pointed out that there was a lot more to the functional shape and biomechanics of the human foot than just its proportions.
Paul O'Higgins, professor of anatomy at the Hull York Medical School, UK, said: "The results are quite exciting and will doubtless spur further testing and additional work."